Cyclic sulfur allotropes and routes for their synthesis from sulfur-containing moieties have been described in the literature. For example, cyclododecasulfur, also referred to herein as S12, is known to be present in thermally equilibrated sulfur allotrope mixtures in concentrations dependent on the equilibration temperature, ranging from about 0.39 wt % to 0.49 wt % between 116° C. and 387° C. (see Steudel, R.; Strauss, R.; Koch, L., “Quantitative HPLC Analysis and Thermodynamics of Sulfur Melts”, Angew. Chem. Int. Ed. Engl., 24(1), 1985, pp. 59-60).
Steudel et al describe a method for S12 synthesis in which cyclooctasulfur, also known as S8, is heated to an equilibration temperature of 200° C., cooled to 140° C., and quenched in liquid nitrogen. S12 is recovered from the solid allotrope mixture by multiple extractions, recrystallizations, decantations, and filtrations from very cold carbon disulfide with an overall yield on the sulfur fed of slightly over 0.21 percent. The melting point of the purified S12 is reported as 146-148° C., the generally quoted melting point of purified S12. (Steudel, R.; Mäusle, H-J., “Detection of Large-Ring Sulfur Molecules in Liquid Sulfur: Simple Preparation of S12, α-S18, S20 from S8”, Angew. Chem. Int. Ed. Engl., 18(2), 1979, pp. 152-53; and, Steudel, R.; Eckert, B., “Solid Sulfur Allotropes”, Topics in Current Chemistry (2003) 230, pp. 1-79).
Schmidt and Block describe a method for the synthesis of S12 in which sulfur is heated at 200° C. for 10 min and quenched in water. The resulting solids are stirred for 12 hours with a 6:1 mass ratio of CS2 at room temperature, followed by filtration of insoluble polymeric sulfur, concentration of mother liquor, and recrystallization of crude S12 from the remaining liquor at −30° C. The remaining S8 is dissolved out of the S12 solids with CS2, and S12 crystals are dried. The dried S12, at a yield of 0.1% of the feed S8, has a melting point of 140-142° C., with a higher melting point of 146-148° C. after recrystallization from benzene. (Schmidt, M.; Block, H.-D., “Occurrence of Cyclododecasulfur Compound in Sulfur Melts”, Angew. Chem. Int. Ed. Engl., 6(11), 1967, pp. 955-56).
Mäusle and Steudel describe a cyclic sulfur allotrope synthesis method in which dichlorodisulfide dissolved in CS2 reacts with aqueous solutions of potassium iodide to form unstable diiododisulfide and potassium chloride, which spontaneously decomposes into a mixture of even number homocyclic rings S6, S8, S10, S12, S18, and larger, and 12. Typical yields are 36% S6 and about 1 to 2% 812. (Mäusle, H. J.; Steudel, R., “Simple preparation of Cyclohexasulfur (S6) from dichlorodisulfane (S2Cl2) and ionic iodides”, Z. Anorg. Allg. Chem. 463, 1980, pp. 27-31).
Yet another approach to the synthesis of S12 is described by Schmidt and Wilhelm (Schmidt, M.; Wilhelm, E., “Cyclodocecasulfur, S12”, Angew. Chem. Int. Ed. Engl., 5(11), 1966, pp. 964-65). This method includes the metathesis of dichlorosulfides with polysulfanes, with corresponding generation of by-product HCl:Cl2Sx+H2Sy→2HCl+S12 with x+y=12
Schmidt and Wilhelm combine dropwise mixtures of S4Cl2 in CS2 and H2S8 in CS2 into a mixture of diethylether and CS2 over 25 hours. After twelve hours, crude Sit crystals are filtered off periodically. The resultant crude S12 is redissolved in CS2 held at 40° C., and recrystallized by concentration of the crude S12—CS2 solution. Final recrystallization is from benzene, with an overall S12 yield of 15% to 20% based on the sulfur fed.
Yet another method for cyclic sulfur allotrope synthesis involves the reaction of a sulfur transfer agent, bis(π-cyclopentadienyl)-titanium(IV) pentasulfide, (titanocene pentasulfide) or (C5H5)2Ti(S5) with sulfur dichloride (SCl2) to form titanocene dichloride, S6, and some S12. In this method, (C5H5)2Ti(S5) in CS2 is treated with SCl2 in CS2 at 0° C. The filtrate containing S6 and S12 is filtered from the titanocene dichloride precipitate and evaporated to give an orange-yellow precipitate. S6 is dissolved with cold CS2 and the remaining solids are dissolved in hot CS2. S12 is recovered by cooling and crystallization from the final CS2 solution. The overall sulfur yield is 87% to S6 and 11% to S12 (see Schmidt, M.; Block, B.; Block, H. D.; Köpf, H.; Wilhelm, E., “Cycloheptasulfur, S7, and Cyclodecasulfur, S10—Two New Sulfur Rings”, Angew. Chem. Int. Ed. Engl., 7(8), 1968, pp. 632-33).
Prior art methods for manufacturing cyclic sulfur allotropes all suffer from one or more drawbacks such as low yields, multiple convoluted manufacturing steps, expensive, complex, and limited-availability starting materials and intermediates and tedious isolation and purification of end products. Most work in this field has accordingly been limited to academic endeavor, and commercially acceptable methods for cost-effective, efficient large-scale production have not heretofore been reported. A continuing need therefore exists for a robust, high-yield, safe, and cost-effective method for the manufacture of cyclic sulfur allotropes, and specifically cyclododecasulfur, that meets industrial criteria for commercial implementation.